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Re: PATCH: PR target/40470: unable to find a registe r to spill in class ‘SSE_FIRST_REG’

On Wed, Jun 17, 2009 at 5:05 PM, Dave
Korn<> wrote:
> H.J. Lu wrote:
>> On Wed, Jun 17, 2009 at 11:34 AM, Jeff Law<> wrote:
>>> H.J. Lu wrote:
>>>> On Wed, Jun 17, 2009 at 11:18 AM, Vladimir Makarov<>
>>>> wrote:
>>>>> I am agree with Jeff and Richard. ?There is one more reason to avoid
>>>>> using
>>>>> hard registers. ?Usage of hard registers tends to create more spill
>>>>> failures
>>>>> in reload.
>>>> It is not like you have a choice here. The register for those insns is
>>>> fixed.
>>>> Sooner or later you have to allocate xmm0 for them.
>>> And how is that different from any other port that has insns which require
>>> specific registers for particular insns. ?This is nothing new or uncommon.
>> Have you compared generated codes on such insns with and
>> without early hard register assignment? My observations are
>> early hard register assignment improves RA on insn with
>> fixed hard registers:
> ?It is very possible there is a real problem in this area. ?I recently(*) was
> investigating the code generated by an inline asm which used the x86-specific
> "a" constraint to force one of the operands into %eax. ?I found that the
> compiler generated a seemingly pointless spill-and-restore (effectively it
> combined a dead store with a nop move!) unless I used a register asm to force
> the operand into %eax early. ?This code:

That matches my observation. I don't know where the real
problem is.


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